Apparatus for projecting an electron beam along a curved path having variable impedance



MTRQQ 1965 E R. ANDERSON ETAL 3,204,096

APPARATUS FOR PROLTEGTING AN ELECTRON BEAM ALONG A CURVED PATH HAVING VARIABLE IMPEDANCE Filed July 9, 1952 ICT/F/IE l0 .JV (mama/r Cuiinvr INVENTOR. I'M/"r7 P. 44 019149 vain/mm:

prrazvm United States Patent 3,204,096 APPARATUS FOR PROJECTING AN ELECTRON BEAM ALONG A CURVED PATH HAVING VARIABLE IMPEDANCE Emmett R. Anderson, Berkeley, and Charles W. Hanks, Orinda, Calif., assignors, by mesne assignments, to Stautfer Chemical Company, New York, N.Y., a corporation of Delaware Filed July 9, 1962, Ser. No. 208,504

4 Claims. (Cl. 250-496) This invention relates to apparatus for projecting an electron beam along a curved path, and, more particularly, to means for controlling the curved trajectory of an electron beam in an electron beam furnace in order to guide it'to impinge upon and bombard a material to be heated.

When material is melted in electron beam furnaces, there is unavoidably generated a considerable amount of gases and vapors. Preferably a high-vacuum evacuation system is provided to remove such gases as rapidly as possible but, as in the case of entrapped gases which are released in bursts, the presence of a certain amount of gas and vapor ions, particularly in the melt zone, is unavoidable. If the gas ions invade the zone in which the electron guns are located a considerable amount of ditficulty may result, not the least of which is the creation of a short circuiting flow of electrons between gas ions in the discharge field between the cathode and anode of the electron gun. Consequently, it is preferred to space the electron guns some distance from the mold or melt zones and to employ a magnetic field for guiding and focusing the electron beams along curved paths to their remote targets.

In operation of such electron beam furnaces, power may be supplied to an electron-emissive cathode from a constant current source so that, when the cathode filament is heated to emit electrons at the desired rate and ideal conditions of equilibrium otherwise obtain, there is a constant discharge voltage of a desired value and, hence, a closely controlled acceleration and projection of the emitted electrons under steady-state equilibrium conditions. However, there are inevitably variations in the impedance of the electron beam path resulting from various conditions. such as variations in the quantity and distribution of ionized matter within an operating furnace. Since the discharge current is kept relatively constant by the constant current source, these impedance variations cause corresponding variations in the discharge voltage, and, hence, variations in the velocities of the electrons accelerated by the electron gun. trajectgrypf th 1 electron ve ocit I: l

agnetic field a variation eratifig otentimm f the magnetic field inherentl resul uctuatio of tliEYlEEFon earn tra'ector such that the beam ay strike, emmer sam flmtmimf It it, therefore, an object of this invention to provide electric circuitry for maintaining a desired trajectory of a magnetically deflected electron beam in a furnace.

It is a further object of this invention to compensate for any variation in accelerating voltage of electron gun by introducing a corresponding variation in the strength of the magnetic field influencing the path of the electron beam projected from the gun.

Other objects and advantages of this invention will become apparent from the specification following when read in conjunction with the accompanying drawing which comprises:

A schematic illustration of an electron beam furnace Since th d is controm b gl a ii ig 3,204,096 Patented Aug. 31, 1965 with a circuit diagram for controlling operation of the furnace components.

Referring now to the drawing in greater detail, the electron beam furnace 1, which is shown in greatly simplified and partly schematicform, includes a furnace envelope 2 which is evacuated through ducts 4 by a conventional vacuum pump system or other vacuum source 6. For purposes of simplified description the electron beam target is shown merely as a molten pool 8 on top of material 9 contained within a mold or crucible 10.

The molten pool 8 is heated by bombardment of an electron beam 12 projected from an electron gun 14 under the influence and guidance of a magnetic field set up by an electromagnet 16. The components of theelectron g ionically-emissive cathode 18, a focusing electrode 19, and an accelerating electrode or anode 20 which accelerates the electrons and projects them to a remote target, specifically, the molten pool 8. A substantial current supplied by leads 22 from the secondary 23 of the filament transformer 24 is passed through the cathode filament 18 in order to heat it to suitable electron-emitting temperature. The electron-accelerating anode 20 is connected by means of a conductor 25 to the furnace envelope, which is grounded as indicated at28. A substantial negative voltage, for example, in the order of 15 kv. is supplied to the filament 18 and the focusing electrode connected thereto at 26. With the filament being maintained at a high negative potential relative to the anode 20, the'rmionically emitted electrons are drawn from the cathode 18 at high velocity to bombard the pool 8 of molten material in the crucible 10. Magnet 16 has a U-shaped core, with a winding 30 on the center leg between the two parallel legs that form opposed pole faces on opposite sides of gun 14. One pole face is shown at 34, the other has been cut away in the section view. 'The coil 30 of the magnet 16 is energized by direct current through conductors 31 from a suitable rectifier 32 to generate a magnetic field with lines of force directed into the plane of the drawing toward the pole face 34, transverse to the electron beam, to

deflect the electron beam 12 along a curved path extending over and down onto the molten pool 8.

Since the rti tra' c or of the am means for varying the s of the ma netic field automatica y variations or fluctuations in eectron velocity, to keep the electron beam the same.

In the embodiment of this invention illustrated, both the filament 18 and the magnet coil 30 through rectifier 32, are supplied with current, the magnitude of which is controlled by the output voltage of a constant current network, which varies as a direct function of the electron beam impedance.

The source of power for the system preferably is a three-phase, sixty-cycle commercial source 40 which is first supplied to a conventional, three-phase constantcurrent network 42. Output leads 44, 45 and 46 of the constant-current network are connected to the primaryof a three-phase, step-up transformer 48, the secondary of which is connected to a suitable rectifier 50. The positive output terminal of the rectifier 50 is connected to ground at 51 and the negative output voltage is supplied to a conductor 52. In a typical application, a voltage of trajectory substantially 12 1S ependent upon both the velocity of the electrons in the ea an t e s reng of the mag 'c'field inii cing it, an varia' vo tage e- WWW-71 tween t e catho e an pro uce g corre- I spon ing variation in electron velocity, resulting in a 480 volts may be supplied at the source 40 and a negative direct current voltage of approximately 15 kv., at steadystate conditions, is supplied by conductor 52 to. the secthat any change in the output voltage of the constantcurrent network 42 will affect both the filament heating current and the current supplied to winding 30 which controls the strength of the magnetic field.

Summarizing the operation of the system, a sdbstantially constant three-phase, sixty-cycle current is delivered over leads 44, 45, and 46 to a step-up transformer, the output of which is delivered to a suitable rectifier 50. From the rectifier with its positive output terminal connected to ground at 51, a negative voltage in the order of i5 kv. is delivered by lead 52 to the secondary winding of the filament transformer 24. Under steady-state equilibrium conditions, the constant current source maintains the cathode l8 and the focusing electrode 19 connected thereto at a constant, high, negative potential, so that there is a constant voltage between the cathode and the electron-accelerating anode 20.

The cathode 18 is heated to emission temperature by means of the single-phase alternating current obtained from output conductors 44 and 45 of the constant-current network, and supplied to the cathode through variable transformer 56 and filament transformer 24. In general, the cathode heating current controls the cathode temperature, which in turn determines the voltage needed to draw from the cathode sufficicnt emission current to match the current supplied to the electron beam by the constant current supply. Thus. increasing the cathode heating current tends to decrease the beam voltage and vice vcrsa. Variable transformer 56 is adjusted to give the desired value of beam voltage under steady-state equilibrium conditions.

When the pool 8 in the crucible 10 is heated, gas and vapor are liberated and become ionized. Some of the ions inevitably drift over toward the electron gun and lower the impedance of the electron beam path in an irregular, fiuctuating manner. With the beam current held relatively constant by the constant-current source, as the impedance varies the beam voltage tends to vary with it, which leads to an erratic, unstable operating condition. More particularly, it is to be noted that the step-up transformer 48 and rectifier 50 couple the output of constant current network 42 in series with the electron beam 12. The series circuit path extends from the negative terminal of the rectifier through conductor 52 and the secondary winding of transformer 24 to the filament 18 of the electron gun 14. From the filament the path extends through the electron beam 12 to the top of the material 9 in crucible 10, and then through the material, crucible, and furnace envelope 2 to the positive terminal of the rectifier 50 via the ground return existing between points 28 and 51. Since the output of the rectifier 50 is the stepped up rectified output of the constant current network 42, the current in the series circuit including the electron beam is constant. From Ohms Law, the beam voltage must vary with the impedance of the beam path inasmuch as the beam current is maintained constant. Consequently, as the impedance fluctuates irregularly, so does the beam voltage. Also, the impedance is decreased if the filament 18 becomes overheated, for example by ion bombardment, causing it to emit an excessive amount of electrons, and this too leads to a loss of beam power and unsatisfactory operation. Either event may lead to breakdown and arcing, loss of power in the beam, and damage to the furnace. These difficulties are overcome as follows: variations in the beam voltage are reflected in like variations in the voltage at the output of constant current network 42. In this regard, the impedance of the beam path is a portion of the output load of the rectifier 50 by virtue of the above-noted series circuit connection therebetween, Likewise, the beam path impedance is a portion of the load presented to the output of the constant current network 42 inasmuch as such output is coupled through the transformer 48 and rectifier 50 to the series circuit which includes the electron beam. Thus, variation in the impedance of the beam path constitutes variation in the load of the constant current network 42. Since the constant current network maintains the load currcnt'at a constant level, the output voltage of the constant current network at leads 44, 45, and 46 necessarily varies in accordance with the impedance of the load, and therefore with the impedance of the beam path. As noted previously, the beam voltage varies directly with the beam path impedance and, thus, it can be said that the output voltage of the constant current network correspondingly varies in accordance with the beam voltage, viz., beam voltage variations are reflected in like variations in the output voltage of the constant current' network. Hence, when the impedance of the beam path drops, there is an almost immediate drop, within about one cycle, in the voltage supplied through leads 54 and 55 and transformers 56 and 24 to filament 18. Thus the filament current is reduced and the cathode begins to cool. Since a small drop in cathode temperature greatly reduces the thermionic emission of electrons from the cathode, this arrangement provides a fast and powerful voltage restoring means for holding the beam voltage close to a desired average value.

Although the arrangement described stabilizes the average bcam voltage at a relatively constant value, there are still short-term voltage swings of substantial amplitude, and hence there are substantial fluctuations in the velocity of the electrons in the beam. 11 focusing field remained c I tro wuweheamaummwd bombard an area considerabl lar er than the surface of poth oft e magnetic 5m the ar' onsi e. invention, such adjustment of the field strength is achieved automatically by supplying current to electromagnet winding from a source of voltage that varies in the same manner as the beam voltage, preferably from the output connections of the constant-current network 42. When the beam voltage drops, the output voltage at leads 44 and of the constant current network drops correspondingly in the manner discussed hereinbefore. Since leads 44 and 45 are connected in energizing relation to the variable transformer 58, the reduced output voltage of the constant current network is applied to such transformer. This effects a decrease in the output voltage of the transformer as applied to the rectifier 32, and therefore in the D.C. output voltage of the rectifier which energizes the coil 30 of electromagnet 16. The flow of current through the coil ishence decreased, resulting in a decrease in the strength of the magnetic focusing field generated by the electromagnet. Conversely, a rise in beam voltage effects a rise in the constant current network output voltage at leads 44 and 45. This rise in constant current network output voltage is applied to the transformer 58, to in turn increase the output voltage applied therefrom to the rectifier 32. The rectifier voltage is thus increased as is therefore the current through the coil 30. This, of course, results in a corresponding increase in the strength of the magnetic focusing field generated by the electromagnet 16. It will thus be appreciatcd that the strength of the magnetic focusing field is c ntinuously varied as a d fplnction of the beam volta ize the trajectory of the beam.

- consideration the magnetization curves of the transformer cores, leaking fluxes, winding resistances, and other circuit impedances; in this manner, a satisfactory match between beam voltage and magnetic field strength may be obtained over a considerable range of voltage variations. For a closer match, if required, more elaborate circuits may be provided for controlling the magnetic current in response to the beam voltage.

While a preferred embodiment of this invention has been illustrated and described herein, it is apparent that modifications and changes therein may be made by those skilled in the art without departing from the principles of this invention.

What is claimed is:

1. Apparatus for projecting an electron beam to a target along a curved path having variable impedance, comprising:

(a) an electron gun including a thermionic cathode and accelerating 'means projecting an electron beam into the desired path;

(b) an electromagnet arranged to Provide a magnetic field transverse to the beam for guiding it along the curved path;

(c) a constant current source and means connecting the source between said cathode and target, whereby the source voltage tends to vary as the impedance of the path varies; and

(d) means energizing said electromagnet with said source voltage to vary magnetic field strength in opposition to variations in beam voltage for maintaining the path of the beam substantially constant.

2. Apparatus for projecting an electron beam along a curved path having variable impedance, comprising:

(a) an electron gun including a thermionic cathode and means projecting an electron beam into the desired path;

(b) an electromagnet arranged to provide a magnetic field transverse to the beam for guiding it along the curved path;

(c) a constant current network having alternating current output connections;

(d) transforming and rectifying means connected to receive alternating current from the output connections of said constant current network and to supply direct current to the electron beam projected by said electron gun, whereby the voltage at the output connections of the constant current network tend to vary as the impedance of the electron beam path varies; and

(e) transforming and rectifying means connected to receive alternating current from the output connections of the same constant current network and to supply direct current to said electromagnet, whereby the strength of the magnetic field is automatically controlled to keep the path of the electron beam substantially the same. i 3. Apparatus as defined in claim 2, additionally comprising transforming means connected to receive alternating current from the output connections of the same constant current network and to supply cathode heating current to said electron gun.

4. An electron beam furnace comprising:

(a) means for supporting a bombardment heated, gas

and vapor evolving target;

(b) a filamentary, thermionically electron-emitting cathode disposed to one side of the target;

(c) means for focusing electrons emitted by said cathode into an electron beam;

(d) an electromagnet having opposed pole faces disposed upon opposite sides of the electron beam to provide a transverse magnetic field for deflecting the beam along a curved path leading to the target;

(e) a constant current, network having alternating current output connections;

(f) a step-up transformer connected to the output connections of said constant current network;

(g) a rectifier connected to said step-up transformer and connected to said cathode and the target for supplying asubstantially constant direct current electron beam between the cathode and the target, whereby the direct current beam voltage and the alternating current voltage at the output connections of the constant-current network tend to vary while the furnace is in operation;

(h) a first variable transformer connected to the output connections of the same constant-current network;

(i) a filament transformer connected to said first variable transformer and connected to supply heating current to said filamentary cathode, whereby the filament heating current varies as the beam voltage varies;

(j) a second variable transformer connected to the output connections of the same constant-current network; and

(k) a rectifier connected to said second variable transformer and connected to supply direct current to said electromagnet, whereby the strentgh of the magnetic field varies as the beam voltage varies.

References Cited by the Examiner UNITED STATES PATENTS 3,068,309 12/62 Hanks 1331 3,087,211 4/63 Howe 13-31 RALPH G. NILSON, Primary Examiner.

RICHARD M. WOOD, Examiner. 

1. APPARATUS FOR PROJECTING AN ELECTRON BEAM TO A TARGET ALONG A CURVED PATH HAVING VARIABLE IMPEDANCE, COMPRISING: (A) AN ELECTRON GUN INCLUDING A THERMIONIC CATHODE AND ACCELERATING MEANSA PROJECTING AN ELECTRON BEAM INTO THE DESIRED PATH; (B) AN ELECTROMAGNET ARRANGED TO PROVIDE A MAGNETIC FIELD TRANSVERSE TO THE BEAM FOR GUIDING IT ALONG THE CURVED PATH; (C) CONSTANT CURRENT SOURCE AND MEANS CONNECTING THE SOURCE BETWEEN SAID CATHODE AND TARGET, WHEREBY THE SOURCE VOLTAGE TENDS TO VARY AS THE IMPEDANCE OF THE PATH VARIES; AND 